> (a) design the API correctly; 
> (b) ship something that definitely works with a proven ThreadPoolExecutor; 

Yury and Andrew, here's my latest API design for asyncio.ThreadPool: https://github.com/python/cpython/compare/master...aeros:asyncio-threadpool. This is for the initial ThreadPoolExecutor version, using the design based on Yury's suggestions. I plan on extending upon the docstrings, writing tests, and the documentation for it.

My idea was to use the new Lib/asyncio/pools.py and AbstractPool to eventually implement an asyncio.ProcessPool (and the native version of asyncio.ThreadPool).

I plan on opening a PR after I finish writing some tests and documentation, I'd like to include it all in the same PR if possible. But let me know what you think about the current API design, it would be much easier for me to make modifications at this stage.